A new way to combine
two materials with special electrical properties—a monolayer superconductor and
a topological insulator—provides the best platform to date to explore an
unusual form of superconductivity called topological superconductivity. The
combination could provide the basis for topological quantum computers that are
more stable than their traditional counterparts.
Superconductors—used in
powerful magnets, digital circuits, and imaging devices—allow the electric
current to pass without resistance, while topological insulators are thin films
only a few atoms thick that restrict the movement of electrons to their edges,
which can result in unique properties. A team led by researchers at Penn State
describe how they have paired the two materials in a paper appearing Oct. 27 in
the journal Nature Materials.
"The future of
quantum computing depends on a kind of material that we call a topological
superconductor, which can be formed by combining a topological insulator with a
superconductor, but the actual process of combining these two materials is challenging,"
said Cui-Zu Chang, Henry W. Knerr Early Career Professor and Associate
Professor of Physics at Penn State and leader of the research team.
"In this study, we
used a technique called molecular beam epitaxy to synthesize both topological
insulator and superconductor films and create a two-dimensional heterostructure
that is an excellent platform to explore the phenomenon of topological
superconductivity."
In previous experiments
to combine the two materials, the superconductivity in thin films usually disappears
once a topological insulator layer is grown on top. Physicists have been able
to add a topological insulator film onto a three-dimensional "bulk"
superconductor and retain the properties of both materials.
However, applications
for topological superconductors, such as chips with low power consumption
inside quantum computers or smartphones, would need to be two-dimensional.
In this paper, the
research team stacked a topological insulator film made of bismuth selenide
(Bi2Se3) with different thicknesses on a superconductor film made of monolayer
niobium diselenide (NbSe2), resulting in a two-dimensional end-product. By
synthesizing the heterostructures at very lower temperature, the team was able
to retain both the topological and superconducting properties.
"In
superconductors, electrons form 'Cooper pairs' and can flow with zero
resistance, but a strong magnetic field can break those pairs," said
Hemian Yi, a postdoctoral scholar in the Chang Research Group at Penn State and
the first author of the paper.
"The monolayer
superconductor film we used is known for its 'Ising-type superconductivity,'
which means that the Cooper pairs are very robust against the in-plane magnetic
fields. We would also expect the topological superconducting phase formed in
our heterostructures to be robust in this way."
By subtly adjusting the
thickness of the topological insulator, the researchers found that the
heterostructure shifted from Ising-type superconductivity—where the electron
spin is perpendicular to the film—to another kind of superconductivity called
"Rashba-type superconductivity"—where the electron spin is parallel
to the film.
This phenomenon is also
observed in the researchers' theoretical calculations and simulations.
This heterostructure
could also be a good platform for the exploration of Majorana fermions, an
elusive particle that would be a major contributor to making a topological
quantum computer more stable than its predecessors.
"This is an
excellent platform for the exploration of topological superconductors, and we
are hopeful that we will find evidence of topological superconductivity in our
continuing work," said Chang. "Once we have solid evidence of
topological superconductivity and demonstrate Majorana physics, then this type
of system could be adapted for quantum computing and other applications."
Reference: Nature Materials
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